The present disclosure relates to an artificial knee joint usable for replacement of damaged or worn out knees. In particular, conditions such as arthritis or rheumatism leads to damage to the knee joint, which can be alleviated by replacement with an artificial joint. Likewise, it is clear that people who suffer from accidents can also benefit from such a replacement joint.
It is typical in the art to provide a knee joint comprising two components. The first component is intended to be attached to the femur of a patient, with the second component being attached to the tibia of the patient. A bearing surface is provided between these two components, such that a lower portion of the femoral component lies on an upper portion of the tibial component and can slide thereon. As is clear, the knee joint requires that the tibia and femur rotate and flex with respect to each other, this is normally achieved by having a curved surface on the outer side of the femoral component. Providing this curved bearing surface on the femoral component, allows the femur to rotate and flex over the bearing surface of the tibial component in much the same way as a normal knee joint functions.
Clearly it is necessary to restrict the amount of relative rotation and flexion possible between the femoral and tibial components, which is often achieved by providing a slot or gap in a portion of the femoral component, and an appropriate post on the tibial component.
Positioning the post within the gap, allows the femoral component to move between two set rotation and flexion stops, and can thus be utilised to mirror and duplicate the normal motion of a knee. That is, the post will restrict the motion of the femoral component to the extent of the gap provided in the femoral component, thus allowing the amount of relative rotation and flexion between the two components to be tailored and appropriately limited.
U.S. Pat. No. 5,549,686 describes a replacement knee joint, in which a post interacts with a gap as described above. In order to allow for high degrees of bending of the knee joint, a femoral cam is provided at the rear-most portion of the femoral component. This femoral cam interacts with the posterior side of the post, and thus can be utilised to allow high angles of bend and rotation between the femoral and tibial components.
A structure which is similar to the above prior art is also described in U.S. Pat. No. 6,123,729. This document further discusses the internal socket within the femoral component for interacting with the end of the femur. Again, the curved structure of the outer surface of the femoral component allows for the relative rotation between the femoral and tibial components, and a femoral cam is provided for interacting with a post on the tibial component to improve the high flexion angle between these two components.
One aspect which is not addressed by either of these prior art replacement knees, is the general asymmetry between the numerous tendons and ligaments present within the knee. It is not unusual for the tendons and ligaments of a patient to have different strengths and lengths, which is particularly apparent at high flexion angles of the knee joint. This will tend to try and pull the orientation of the knee joint out of complete alignment, such that a degree of twist occurs between the tibia and femur of the patient, which then gets translated to the knee joint. The present disclosure relates to a solution to this problem and presents a knee joint suitable for allowing the femoral and tibial components to rotate and/or wobble at very high flexion angles, in particular a controlled relative rotation/wobbling motion.